Coil arrangement for improved field uniformity



March 14, 1967 w, v, sc ETAL 3,309,638

COIL ARRANGEMENT FOR IMPROVED FIELD UNIFORMITY Filed May 7, 1965 2Sheets-Sheet l INVENT'II )RS Mam/ i 1 RAN/$6.11 MPH/me 04405190 March14, 1967 v w. v, RAUSCH ETAL 3,309,638

COIL ARRANGEMENT FOR IMPROVED FIELD UNIFORMITY Filed May '7, 1965 2Sheets-Sheet 2 INVENTORS Mum/'1 1 Ram-c0 41210701? 63440520 m WMJrroxvsrs United States Patent Ofiice 3 ,3ii9,638 Patented Mar. 14, 19673,309,638 COIL ARRANGEMENT FOR IMPROVED FIELD UNIFORMITY William V.Rausch, 1450 N. Larch Lane, Minneapolis,

Minn. 55427, and Arthur Calderon, 2514 Cedarwood Ridge, Minnetonka,Minn. 55343 Filed May 7, 1965, Ser. No. 454,148

5 Claims. (Cl. 335-299) This invention relates to an electromagneticcoil arrangement for improving the uniformity of a magnet c field overthe planar substrate area of ferro-magnetic thin film depositionapparatus and, more particularly, to a pair of series aidingcylin-drically warped elliptical electromagnetic coils positionedpartially around a deposition chamber containing a substrate material toprovide a more uniform magnetic field across the planar surface of thesubstrate material.

In modern computing equipment, memory storage elements often consist ofbi-stable ferro-magnetic thin films which are produced by vacuumdeposition of a vaporized metal onto a heated substrate material in thepresence of an orienting field. During the evaporation process, thesubstrate material is maintained at an elevated temperature, and amagnetic field is applied in the plane defined by the substratematerial. The vaporized metal condenses on the substrate material and isallowed to cool in the presence of the magnetic field. The combinationof heated substrate and the magnetic field (in some presently un-'explained way), is responsible for the establishment of a preferred oreasy axis of magnetization of the thin film. The easy axis ofmagnetization is in the direction of the applied magnetic field, whilethe planar axis perpendicular to the direction of the orienting fiel-d ithe hard axis of magnetization.

Normally, a large number of thin films are simultaneously deposited uponone or more planar substrates. For economic reasons, as large asubstrate area as possible is coated in a single deposition. Thelocation and arrangement of the individual thin film is determined by astencil mask with an open pattern of the desired thin film arrangementwhich is placed over the substrate during deposition. It is essentialfor proper computer operation that the easy axes of all of the bi-stablethin films be substantially parallel. This requires that the directionof the magnetic field across the planar substrate surface be uniformduring deposition. It is preferable to locate the electromagnetic coilsoutside of the deposition chamber to reduce interference with thedeposition process, although this makes' the creation of a uniformmagnetic field more difficult and, consequently, limits the total numberof bi-stable elements that can be deposited in a single depositioncycle, which increases the overall cost of the elements. Large andcomplex electromagnetic coil arrangements have previously been designedwhich will produce a magnetic field of substantially parallel lines offlux across a relatively small planar area. In order that externallysituated circular coils, whose center axes coincide, can create asatisfactory magnetic field across the substrate area, their diametersmust be approximately twice the diameter of the deposition chamber ifthe outside diameter or dimension of the deposition chamber equals thecoil spacing. This means that for a typical deposition chamber of adiameter of two feet, the diameter of each electromagnetic coil must beapproximately four feet. In this case the coils would require anadditional space allotment of about 16 cubic feet. The reason that suchlarge circular coils are required is that the lines of flux establishedbetween them tend to diverge, or bulge out, as the perpendiculardistance from the axes of the coils increases. The width of the areanear the coil axes in which the magnetic field is satisfactorily uniformis proportional to the diameter of the two coils and the spacing betweenthem, the magnetic field in a given area near the axes of large coilsbeing less distorted than when near the axes of small coils, if thespacing of the coils is the same in both cases. In other words, largercircular coils will produce a larger area of uniform lines of fi-ux.However, use of these larger coils is very undesirable for two reasons:first, the design of most commercial evaporators does not allow theinstallation of coils of diameters equal to twice the diameter ordimension of the vacuum chamber, such that the coil co-axis is in theplane of the substrates; and second, installation of large coils wouldmake loading and unloading of the chamber very cumbersome, timeconsuming and costly.

The present invention creates a nearly uniform magnetic field by makingphysical adjustments to a set of series aiding circular coils. In theprocess, circular coils are wound having a diameter equal to thediameter of the deposition chamber. The wire used and the manner ofwinding must leave the coils somewhat flexible to allow subsequentchanges in shape. These circular coils are then flattened into coils ofellipselike shape, hereafter referred to as elliptical coils althoughthey need not be perfectly elliptical, such that the ratio of theoriginal diameter of the circular coil to the minor axis of theresulting elliptical coil is approximately 1.5. The major axis is thenapproximately twice the length of the minor axes. The elliptical coilsare then formed around the bell jar in such a way that the major axis ofeach elliptical coil is in the same plane as the substrate. Experimentalobservation of the effects of the magnetic field on iron filings hasshown that a ratio of the original'dia-meter of the coil to the minoraxis of the intermediate elliptical coil of approximately 15 produces amagnetic field of satisfactory uniformity for use in this filmdeposition processes over large substrate areas (up to 64 sq. in.) whenthe coil is warped around the bell jar. The coils in final configurationmay be termed cylindrically warped elliptical coils.

It is an object of this invention to provide apparatus for producing amagnetic field with nearly parallel lines of flux across a large planarsurface.

It is another object of this invention to provide apparatus forproducing a nearly uniform magnetic field with simple and inexpensiveelectromagnetic coils.

It is a further object of this invention to minimize the weight andoverall volume of the electromagnetic coils.

These and other objects of this invention will become apparent uponconsideration of the accompanying claims,

specification and drawings, of which:

FIG. 1 is a perspective view of a pair of cylindrically warpedelliptical electromagnetic coils of this invention supported around athin film deposition chamber;

FIG. 2 is a second perspective view of the pair of cylrndrically warpedelliptical electromagnetic coils of FIG. 1 supported around a thin filmdeposition chamber;

FIG. 3 is a diagram showing the dimensional relationshrp between adeposition chamber bell jar, original circular coils, and ellipticalcoils prior to being formed around the bell jar.

FIG. 4 is a combined elevational view and top plan view of anelectromagnetic coil of this invention in circular form prior to beingaltered in shape.

FIG. 5 is a combined elevational view and top plan view of anelectromagnetic coil of this invention showing the circular coil of FIG.4 after having been formed into an elliptical coil.

FIG. 6 is a combined perspective view and top plan view of anelectromagnetic coil of this invention showing the elliptical coil ofFIG. 5 after having been formed around a cylindrical surface.

FIG. 7 is a top view of representative electromagnetic J lines of fluxproduced by a pair of series aiding planar circular electromagneticcoils which are shown in sec tion; and

FIG. 8 is a top view of representative electromagnetic lines of fluxproduced by a pair of cylindrically elliptical warped coils of thisinvention which are shown in section.

Referring to the drawings and numerals of reference thereon, in FIGS. 1and 2 a pair of coils 11 and 12 are mounted on a coil support frame 13by a set of brackets 14. Coil support frame 13 is placed over a bell jar15 of a deposition chamber indicated generally at 16. A cover panel ofcoil support frame 13 rests on a platform 17 secured to the top of belljar 15 so as to allow a coaxis 18 of coils 11 and 12 to lie in the planeof a substrate 19 which is supported inside of bell jar 15 on astanchion 44.

Referring to FIGS. 3, 4, 5, and 6 a circular coil 23 is formed into anelliptical coil 24 such that the diameter 25 of the original circularcoil 23 is approximately 1.5 times the minor diameter 26 of theelliptical coil. The diameter 27 of the bell jar 15 is approximatelyequal to the diameter 25 of circular coil 23. After elliptical coil 24has been formed from circular coil'23, it is formed around a cylindricalmandrel, 32 as in FIG. 6, and then placed around bell jar 15, as shownin FIGS. 1 and 2, so that a plane defined by the now curved major axis31 of semicircular coils 11 and 12 will coincide with the plane definedby substrate 19.

In FIG. 7 a set of lines of flux 35 represent a portion of the magneticfield created by a pair of series aiding electromagnetic planar,circular coils 36 and 37. Lines 7 of flux 35 can be seen to deviate froma straight and uniform path and to bulge outwardly, the amount of bulgebeing proportional to the distance from coaxis 38 of coils 36 and 37,and proportional to the coil diameter and spacing.

In FIG. 8 a set of lines of flux 39 represent a corresponding portion ofthe magnetic field created by a pair of series aiding cylindricallywarped elliptical electromagnetic coils 11 and 12 having a coaxis 42.The upper and lower portions of coils 11 and 12 as viewed in FIG. 8, canbe visualized as having effectively countered the tendency of lines offlux 39 to bulge in a direction away from coaxis 42. It is thus apparentthat resultant magnetic field between coils 11 and 12 is more nearlyuniform and unidirectional than in the corresponding area in FIG. 7between coils 36 and 37. It is also evident from FIGURES 1 through 8that the overall bulk of the coils is reduced and that the fieldintensity is increased for the same number of turns and current througheach set of coils.

Although this invention has been described with regard to filmdeposition apparatus, its use is not intended to be thus limited. Thisinvention which allows maximum field uniformity in essentially a planararea for minimum coil bulk, would be useful in numerous other devicessuch as Kerr apparatus, Bitter pattern studies, and hysteresis loopchecking apparatus.

What is claimed is:

1. Apparatus for producing a substantially uniform magnetic field in aplanar area comprising: a pair of series aiding electromagnetic coilshaving the same axis;

said coils being positioned adjacent opposite sides of the planar area;said axis lying in the planar area; each of said coils having a greaterdiameter in a direction parallel to the planar area than in a directionperpendicular to the planar area; and said coils being curved at leastpartially around said opposite sides of the planar area so that asurface defined by each of said coils is perpendicular to the planararea.

2. Apparatus for producing a substantially uniform magnetic field in aplanar area comprising; a pair of series aiding electromagnetic coilshaving the same axis; said coils being positioned adjacent oppositesides of the planar area; each of said coils having a greater diameterin a direction parallel to the planar area than in a directionperpendicular to the planar area; and said coils being curved at leastpartially around said opposite sides of the planar area so that asurface defined by each of said coils is perpendicular to the planararea, and so that the intersection of the surface defined by each ofsaid coils by the plane of the planar area is an arc of a circle.

3. Apparatus for producing a substantially uniform magnetic field in aplanar area comprising: a pair of series aiding electromagnetic coilshaving the same axis, said coils being positioned adjacent oppositesides of the planar area; said axis lying in the plane of the planararea; each of said coils having a greater diameter in a directionparallel to the planar area than in a direction perpendicular to theplanar area; and said coils being curved at least partially around saidopposite sides of said planar area such that each of said coils definesan elliptical cylindrical surface perpendicular to said planar area.

4. The apparatus of claim 3 in which each of said coils are shaped in anellipse such that the diameter of each of said coils when forming acircle is 1.5 times the minor axis of each of said coils when shaped inan ellipse.

5. In apparatus for the production of thin films on a substrate materialwithin a cylindrically shaped deposition chamber, means for producing asubstantially uniform magnetic field across the planar surface of saidsubstrate material, said means comprising: a pair of series aidingelectromagnetic coils having the same axis; said coils being positionedadjacent opposite sides of the substrate material; said axis lying in aplane defined by the substrate material; each of said coils being acylindrically warped ellipse having a major axis and a minor axis; saidminor axes being perpendicular to said plane defined by the substratematerial; said major axes lying in said plane defined by the substratematerial; and said major axes being approximately twice the length ofsaid minor axes.

References Cited by the Examiner UNITED STATES PATENTS 3,065,105 11/1962Pohm 1l7235 3,092,510 6/ 1963 Edelman. 3,131,078 4/1964 Fuller et -all17--93.2 X

BERNARD A. GILHEANY, Primary Examiner,

G. HARRIS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,309,638 March 14, 1967 William V. Rausch et a1.

pears in the above numbered pat- It is hereby certified that error ap ntshould read as ent requiring correction and that the said Letters Patecorrected below.

he printed specification, lines 4 to 6, 1450 N. Larch Lane, Minneapolis,Minn.

55427, and Arthur Calderon, 2514 Cedarwood Ridge, Minnetonka,

h, Minneapolis, Minn. and

Minn. 55343" read William V. Rausc Arthur Calderon, Minnetonka, Minn.assignors to Fabri-Tek Incorporated, Minneapolis, Minn., a corporationof Wisconsin In the heading to t for "William V Rausch,

Signed and sealed this 7th day of November 1967.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, J r.

Commissioner of Patents Attesting Officer

1. APPARATUS FOR PRODUCING A SUBSTANTIALLY UNIFORM MAGNETIC FIELD IN APLANAR AREA COMPRISING: A PAIR OF SERIES AIDING ELECTROMAGNETIC COILSHAVING THE SAME AXIS; SAID COILS BEING POSITIONED ADJACENT OPPOSITESIDES OF THE PLANAR AREA; SAID AXIS LYING IN THE PLANAR AREA; EACH OFSAID COILS HAVING A GREATER DIAMETER IN A DIRECTION PARALLEL TO THEPLANAR AREA THAN IN A DIRECTION PERPENDICULAR TO THE PLANAR AREA; ANDSAID COILS BEING CURVED AT LEAST PARTIALLY AROUND SAID OPPOSITE SIDES OFTHE PLANAR AREA SO THAT A SURFACE DEFINED BY EACH OF SAID COILS ISPERPENDICULAR TO THE PLANAR AREA.